1. Field of the Invention
This invention relates to an electrophoretic composition and to an image display medium using the silicone-oil soluble polymer. The present invention is also directed to an image display device using the image display medium. More particularly the present invention relates to an electrophoretic composition which can reversibly form an image of white and/or colored particles contained therein and moveable upon application of an electric field.
2. Discussion of the Prior Art
Display devices such as cathode ray tubes (CRTs) and liquid crystal displays (LCDs) have been used as an image display terminal for displaying images such as letters, static images and moving images. These devices can instantaneously display images (i.e., digital data) and rewrite the images. However, these devices have various drawbacks. For, example, it is a hard work to always carry such devices. In addition, to see such a display device for a long period of time causes eye fatigue. Further, when such a display device displaying images is turned off, the images are disappeared.
When images such as letters and static images are distributed or stored as a document, the images are typically recorded by a printer on a paper medium to form a hardcopy. Images on such a hardcopy are easier to read than those in the display devices mentioned above. In addition, when reading images on hardcopies, the degree of eye fatigue is much less than that when reading images in the display devices. Further, a reader can see images on hardcopies while having a free posture. Furthermore, hardcopies can be easily carried because of having lightweight. However, hardcopies have a problem in view of material saving such that much labor and costs are necessary to recycle used hardcopies.
Therefore, a need has been existed for a paper-like image display medium having a combination of the advantages of the above-mentioned display devices and the advantages of hardcopies. Until now, display media using a liquid crystal dispersed in a polymer, a bistable cholesteric liquid crystal, an electrochromic material or an electrophoretic material have been proposed. These media are a reflection type and therefore can display high contrast images. In addition, the media can be used as a memorable display medium (i.e., the media have an image maintaining ability even when the power is off).
Among these media, devices using an electrophoretic material have good display qualities and low power consumption and therefore are advantageous over other media. A typical electrophoretic display medium is disclosed in Japanese Laid-Open Patent Publication No. 5-173194 or Japanese Patent No. 2,612,472. In the electrophoretic display medium, an electrophoretic liquid containing electrophoretic particles having a color and dispersed in a dispersion medium having a different color is contained in a cell formed by a pair of first and second electrodes. When a voltage having a polarity opposite to the charge of the electrophoretic particles is applied to the first electrode, the particles move toward the first electrode, resulting in deposition of the particles on the first electrode. To the contrary, when a voltage having the same polarity as the charge of the electrophoretic particles is applied to the first electrode, the particles move toward the second electrode, so that the color of the dispersing medium can be seen from the side of the first electrode. Thus, images can be reversibly displayed.
It is well known that the dispersion of the electrophoretic particles can be stabilized by electrostatic repulsion effect and a stereo effect (i.e., a so-called adsorption layer effect). With respect to the electrostatic repulsion effect, a DLVO theory is established. In this theory, the thickness of the electric double layer and the phase boundary potential (i.e., the so-called xcex6 (zeta) potential) are important factors. Therefore, ions for forming such an electric double layer and a phase boundary potential are needed. Several investigations have been made for aqueous dispersion in which ions can be present.
With respect to the stereo effect, a theory corresponding to the DLVO theory has not yet been established. An investigation using a non-aqueous dispersion system (e.g., petroleum solvent dispersion systems) is reported by F. A. Waite in J. Oil Col. Chem. Assoc., 54, 342 (1971). This investigation relates to a fundamental method for manufacturing a stable non-aqueous solvent dispersion. In this method, a block or graft copolymer which has both a component having an affinity for the particles dispersed in a dispersing medium and another component soluble in the dispersing medium is used.
Japanese Patent Publication No. S40-7047 discloses a method for manufacturing a dispersion, which utilizes the above-mentioned method. In the method, methyl methacrylate is radically polymerized in a hydrocarbon solvent in the presence of a degradable rubber to prepare a stable polymethyl methacrylate dispersion. It is considered that methyl methacrylate is graft-copolymerized with the degradable rubber because it is unlikely that the polymethyl methacrylate particles adsorb the degradable rubber. In addition it is considered that when the graft copolymer is included in a display dispersion, the insoluble part of the graft copolymer associates with the surface of particles therein, and the soluble part of the graft copolymer has a stereo effect. Thus, the dispersion stability of the particles can be maintained.
However, the stability of the electrophoretic particles in a dispersion using a non-aqueous solvent is not sufficiently high. Namely, there is a limitation in service life of the image display medium in which such a dispersion is used.
The present inventor investigated a non-polar aprotic solvent type dispersion including (a) an organic material having an acidic group but not having a basic group, (b) an organic material having a basic group but not having an acidic group and (c) an organic material which is compatible with the non-polar aprotic solvent and which has a nonionic polar group (either one of the components (a) and (b) may be copolymerized with the component (c)). As a result of the investigation, the following findings were obtained. Namely, the components (a) and (b) cause an acid-base ionic dissociation in the non-polar solvent with the simultaneous occurrence of salvation (i.e., ion-dipole interaction). When the components (a), (b) and (c) are present in the non-ionic solvent, ions can be stably present in the non-polar aprotic solvent because of an acid-base dissociation of components (a) and (b) associated with the solvation of the polar group of the component (c). In addition, when solid particles such as pigments and metal oxide particles are present in a system including the components (a), (b) and (c), the particulate material can be dispersed more stably. The reason is considered to be that the acidic or basic group of the components (a) and (b) is combined with the particulate material by chemical bonding, adsorption, etc., and fixed thereon. Thereby ionic dissociation occurs at the interface between the surface of the particulate material and the solvent through the solvated component (c). As a result, the particulate material is uniformly charged positively or negatively and can be stably dispersed by the synergy effect of this electrostatic effect and a stereo effect. In Japanese Laid Open Patent Publication No. 2002-62545, published Feb. 28, 2002, the present inventor has proposed an electrophoretic liquid containing a petroleum type solvent (non-polar aprotic solvent) in which charged electrophoretic particles are dispersed.
It has been found that the known electrophoretic liquid as proposed in Japanese Laid Open Patent Publication No. 2002-62545 has a problem because a high intensity electric field should be applied in order to obtain a desired speed of migration of the electrophoretic particles (display speed).
The present invention has been made to solve the above-mentioned problem.
In accordance with one aspect of the present invention an electrophoretic composition in the form of a dispersion comprising a dispersion medium, and electrophoretic particles dispersed in said dispersion medium and having a color selected from white and non-white colors,
said dispersion medium comprising:
(a) a non-polar solvent,
(b) a sulfonic acid compound soluble at least partly in said non-polar solvent, and
(c) a polymeric material soluble in said non-polar solvent.
Because of the presence of the polymeric material, the electrophoretic particles can be stably dispersed in the non-polar solvent. Further, the sulfonic acid compound is dissociated in the solvent to cause the electrophoretic particles to be charged. As a consequence, the migration speed of the electrophoretic particles is high enough even at a low electric field.
In another aspect the present invention provides an image display medium comprising a pair of opposing electroconductive layers defining a gap therebetween, and the above electrophoretic composition filled in said gap, at least one of said electroconductive layers being light-transmissive.
The present invention also provides an image displaying device comprising the above image displaying medium.
It is an object of the present invention to provide an electrophoretic composition which has improved dispersion stability and improved migration speed of the electrophoretic particles.
Another object of the present invention is to provide an image display medium which can maintain displayed images for a long period of time and which has improved responsibility to an applied voltage.